(1) Field of the Invention
This invention relates to the application of a fusion bonded coating to the interior surface of tubular goods.
(2) Description of the Prior Art
Processes for coating the interior surface of tubular goods with fusion bonded thermosetting and thermoplastic coating material were known before this invention. Prior to filing this application, a search of United States Patent and Trademark Office references developed the following U.S. patent references:
DE HART, U.S. Pat. No. 3,207,618; INAMURA ET AL., U.S. Pat. No. 3,850,660; INAMURA ET AL., U.S. Pat. No. 3,974,306; KATO ET AL., U.S. Pat. No. 3,982,050; GIBSON, U.S. Pat. No. 4,089,998; GIBSON, U.S. Pat. No. 4,243,699; WARREN ET AL., U.S. Pat. No. 4,382,421; GIBSON, U.S. Pat. No. 4,420,508.
Despite the advance in the art represented by these patents, problems of uneven coating thickness over the length of the tubular goods, and the presence of "holidays" in the coating, have persisted. The term "holidays" is commonly used in the art to describe those positions in the coating along the length of tubular goods where the bare metal of the pipe wall is exposed by a "gap" or "hole" in the coating.
The frequency of holidays is also related to the thickness of the coating, such that an increase in the coating thickness tends to reduce the number of holidays. However, increasing the coating thickness uses more coating material and increases the cost of the product. A thick coating also reduces the flexibility of the coated tubular goods.
GIBSON, U.S. Pat. No. 4,089,998 applies a vacuum to a discharge end of a length of pipe, and connects the other end to a fluidized bed of coating material. The vacuum withdraws an amount of coating material into the preheated pipe. The pipe is rotated at 80-100 rpm during coating process. Immediately after the coating charge is loaded into the pipe, the vacuum is shut off and air pressure is applied at the load end of the pipe behind the charge to blow the short envelope of coating material through the pipe to the discharge end.
The changes in pressure within the pipe during this coating process also cause changes of the velocity of the coating particles. A thick coating at the load end results where the particles have a long dwell or residence time upon initial loading. Just downstream of the load end, a zone of reduced coating thickness results from the sudden increase in particle velocity. A zone of increasing coating thickness toward the discharge end of the pipe results as the velocity of the particles through the pipe is reduced due to friction and decreased pressure. If increased air pressure is used to compensate for this, the powder particles will have a greater velocity and would tend to be blown through the discharge end without sufficient residence time to melt on the pipe wall. This would produce another zone of decreased film thickness. The number of holidays tends to increase within the zone of decreased coating or film thickness.
GIBSON U.S. Pat. No. 4,089,998 also discloses an alternate structure for coating the pipe, wherein a continuous airflow is moved linearly from a horizontal tube through a pipe rotated at 80-100 rpm. A charge of the powdered coating material is collected in a chamber below the horizontal tube. Air flow upward through the chamber and a vertical tube connecting the chamber to the horizontal tube blows the charge into the horizontal air stream. After the charge is loaded, the vertical injection air stream is stopped. GIBSON discloses shutting off the horizontal air stream before the coating process is complete. The many changes in pressure and airflow and the consequent changes in particle velocity result in varying coating thicknesses throughout the length of pipe.
DE HART uses a vacuum to withdraw a measured charge of coating material from a fluidized bed into one end of a pipe. The vacuum is removed when the remaining portion of the charge is at the middle of the pipe. The process is repeated at the other end of the pipe, either from a second fluidized bed, or by turning the pipe around and repeating with another charge from the first bed. When initially withdrawn into the end of the pipe, the particles tend to be accelerated by the vacuum, resulting in a lower coating thickness at the ends of the pipe. A higher thickness, due to overlap of the coatings, results at the middle of the pipe.
KATO ET AL meter a charge of coating particles into a linear air stream flowed through a short length of small diameter conduit. Deposit of particles on the conduit walls will reduce the amount of powder in the air stream, and constant pressure on the decreasing particle density will cause the suspended particles to increase velocity. This results in a thick coating at the load end that gets thinner as the distance from the load end increases.
In the exercise of greater than ordinary skill, the workers in the art have tried the many different approaches above. They have tried variations in pressure, or the maintenance of constant pressure, with differing effects. Yet, no prior art process has satisfactorily solved the nonuniform coating thickness problems or reduced the occurrence of holidays to an economically profitable level for the coater.